High resistivity silicon-on-insulator substrate comprising an isolation region

ABSTRACT

A multilayer composite structure and a method of preparing a multilayer composite structure are provided. The multilayer composite structure comprises a semiconductor handle substrate having a minimum bulk region resistivity of at least about 500 ohm-cm and an isolation region that impedes the transfer of charge carriers along the surface of the handle substrate and reduces parasitic coupling between RF devices.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a divisional application of U.S. application Ser.No. 15/623,519, which was filed Jun. 15, 2017, and which is herebyincorporated by reference as if set forth in its entirety. U.S.application Ser. No. 15/623,519 claims the benefit of U.S. ProvisionalApplication Ser. No. 62/353,301, filed Jun. 22, 2016. The priorityprovisional application is incorporated by reference herein as if setforth in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to the field of semiconductorwafer manufacture. More specifically, the present invention relates to amethod of preparing a handle substrate for use in the manufacture of asemiconductor-on-insulator (e.g., silicon-on-insulator) structure.

BACKGROUND OF THE INVENTION

Semiconductor wafers are generally prepared from a single crystal ingot(e.g., a silicon ingot) which is trimmed and ground to have one or moreflats or notches for proper orientation of the wafer in subsequentprocedures. The ingot is then sliced into individual wafers. Whilereference will be made herein to semiconductor wafers constructed fromsilicon, other materials may be used to prepare semiconductor wafers,such as germanium, silicon carbide, silicon germanium, gallium arsenide,and other alloys of Group III and Group V elements, such as galliumnitride or indium phosphide, or alloys of Group II and Group VIelements, such as cadmium sulfide or zinc oxide.

Semiconductor wafers (e.g., silicon wafers) may be utilized in thepreparation of composite layer structures. A composite layer structure(e.g., a semiconductor-on-insulator, and more specifically, asilicon-on-insulator (SOI) structure) generally comprises a handle waferor layer, a device layer, and an insulating (i.e., dielectric) film(typically an oxide layer) between the handle layer and the devicelayer. Generally, the device layer is between 0.01 and 20 micrometersthick, such as between 0.05 and 20 micrometers thick. Thick film devicelayers may have a device layer thickness between about 1.5 micrometersand about 20 micrometers. Thin film device layers may have a thicknessbetween about 0.01 micrometer and about 0.20 micrometer. In general,composite layer structures, such as silicon-on-insulator (SOI),silicon-on-sapphire (SOS), and silicon-on-quartz, are produced byplacing two wafers in intimate contact, thereby initiating bonding byvan der Waal's forces, followed by a thermal treatment to strengthen thebond. The anneal may convert the terminal silanol groups to siloxanebonds between the two interfaces, thereby strengthening the bond.

After thermal anneal, the bonded structure undergoes further processingto remove a substantial portion of the donor wafer to achieve layertransfer. For example, wafer thinning techniques, e.g., etching orgrinding, may be used, often referred to as back etch SOI (i.e., BESOI),wherein a silicon wafer is bound to the handle wafer and then slowlyetched away until only a thin layer of silicon on the handle waferremains. See, e.g., U.S. Pat. No. 5,189,500, the disclosure of which isincorporated herein by reference as if set forth in its entirety. Thismethod is time-consuming and costly, wastes one of the substrates andgenerally does not have suitable thickness uniformity for layers thinnerthan a few microns.

Another common method of achieving layer transfer utilizes a hydrogenimplant followed by thermally induced layer splitting. Particles (atomsor ionized atoms, e.g., hydrogen atoms or a combination of hydrogen andhelium atoms) are implanted at a specified depth beneath the frontsurface of the donor wafer. The implanted particles form a cleave planein the donor wafer at the specified depth at which they were implanted.The surface of the donor wafer is cleaned to remove organic compounds orother contaminants, such as boron compounds, deposited on the waferduring the implantation process.

The front surface of the donor wafer is then bonded to a handle wafer toform a bonded wafer through a hydrophilic bonding process. Prior tobonding, the donor wafer and/or handle wafer are activated by exposingthe surfaces of the wafers to plasma containing, for example, oxygen ornitrogen. Exposure to the plasma modifies the structure of the surfacesin a process often referred to as surface activation, which activationprocess renders the surfaces of one or both of the donor water andhandle wafer hydrophilic. The surfaces of the wafers can be additionallychemically activated by a wet treatment, such as an SC1 clean orhydrofluoric acid. The wet treatment and the plasma activation may occurin either order, or the wafers may be subjected to only one treatment.The wafers are then pressed together, and a bond is formed therebetween. This bond is relatively weak, due to van der Waal's forces, andmust be strengthened before further processing can occur.

In some processes, the hydrophilic bond between the donor wafer andhandle wafer (i.e., a bonded wafer) is strengthened by heating orannealing the bonded wafer pair. In some processes, wafer bonding mayoccur at low temperatures, such as between approximately 300° C. and500° C. In some processes, wafer bonding may occur at high temperatures,such as between approximately 800° C. and 1100° C. The elevatedtemperatures cause the formation of covalent bonds between the adjoiningsurfaces of the donor wafer and the handle wafer, thus solidifying thebond between the donor wafer and the handle wafer. Concurrently with theheating or annealing of the bonded wafer, the particles earlierimplanted in the donor wafer weaken the cleave plane.

A portion of the donor wafer is then separated (i.e., cleaved) along thecleave plane from the bonded wafer to form the SOI wafer. Cleaving maybe carried out by placing the bonded wafer in a fixture in whichmechanical force is applied perpendicular to the opposing sides of thebonded wafer in order to pull a portion of the donor wafer apart fromthe bonded wafer. According to some methods, suction cups are utilizedto apply the mechanical force. The separation of the portion of thedonor wafer is initiated by applying a mechanical wedge at the edge ofthe bonded wafer at the cleave plane in order to initiate propagation ofa crack along the cleave plane. The mechanical force applied by thesuction cups then pulls the portion of the donor wafer from the bondedwafer, thus forming an SOI wafer.

According to other methods, the bonded pair may instead be subjected toan elevated temperature over a period of time to separate the portion ofthe donor wafer from the bonded wafer. Exposure to the elevatedtemperature causes initiation and propagation of cracks along the cleaveplane, thus separating a portion of the donor wafer. The crack forms dueto the formation of voids from the implanted ions, which grow by Ostwaldripening. The voids are filled with hydrogen and helium. The voidsbecome platelets. The pressurized gases in the platelets propagatemicro-cavities and micro-cracks, which weaken the silicon on the implantplane. If the anneal is stopped at the proper time, the weakened bondedwafer may be cleaved by a mechanical process. However, if the thermaltreatment is continued for a longer duration and/or at a highertemperature, the micro-crack propagation reaches the level where allcracks merge along the cleave plane, thus separating a portion of thedonor wafer. This method allows for better uniformity of the transferredlayer and allows recycle of the donor wafer, but typically requiresheating the implanted and bonded pair to temperatures approaching 500°C.

The use of high resistivity semiconductor-on-insulator (e.g.,silicon-on-insulator) wafers for RF related devices such as antennaswitches offers benefits over traditional substrates in terms of costand integration. To reduce parasitic power loss and minimize harmonicdistortion inherent when using conductive substrates for high frequencyapplications it is necessary, but not sufficient, to use substratewafers with a high resistivity. Accordingly, the resistivity of thehandle wafer for an RF device is generally greater than about 500Ohm-cm. Positive charges in the buried oxide layer (BOX) in asemiconductor on insulator structure and electric fields of operatingdevices induce inversion and accumulation charges under the frontsurface of the handle substrate of the semiconductor on insulatorstructure. The layers formed of these charges provide effective pathsfor the parasitic coupling between RF devices that enhances devicecross-talking, power dissipation, and harmonic distortion of RF signals.

SUMMARY OF THE INVENTION

The present invention is directed to a method of preparing a multilayerstructure, the method comprising: implanting As⁺ ions through a frontsurface of a single crystal semiconductor handle substrate to therebyform an isolation region comprising arsenic-doped semiconductor, whereinthe single crystal semiconductor handle substrate comprises two major,generally parallel surfaces, one of which is the front surface of thesingle crystal semiconductor handle substrate and the other of which isa back surface of the single crystal semiconductor handle substrate, acircumferential edge joining the front and back surfaces of the singlecrystal semiconductor handle substrate, a central plane between thefront surface and the back surface of the single crystal semiconductorhandle substrate, and a bulk region between the front and back surfacesof the single crystal semiconductor handle substrate, wherein the singlecrystal semiconductor handle substrate has a minimum bulk resistivity ofat least about 500 ohm-cm; depositing a dielectric layer on the frontsurface of the single crystal semiconductor handle substrate; andbonding a front surface of a single crystal semiconductor donorsubstrate to the dielectric layer of the single crystal semiconductorhandle substrate to thereby form a bonded structure, wherein the singlecrystal semiconductor donor substrate comprises two major, generallyparallel surfaces, one of which is the front surface of thesemiconductor donor substrate and the other of which is a back surfaceof the semiconductor donor substrate, a circumferential edge joining thefront and back surfaces of the semiconductor donor substrate, and acentral plane between the front and back surfaces of the semiconductordonor substrate, and further wherein the semiconductor donor substratecomprises a cleave plane.

The present invention is further directed to a method of preparing amultilayer structure, the method comprising: implanting O⁺ ions througha front surface of a single crystal semiconductor handle substrate tothereby form an isolation region comprising semiconductor oxide, whereinthe single crystal semiconductor handle substrate comprises two major,generally parallel surfaces, one of which is the front surface of thesingle crystal semiconductor handle substrate and the other of which isa back surface of the single crystal semiconductor handle substrate, acircumferential edge joining the front and back surfaces of the singlecrystal semiconductor handle substrate, a central plane between thefront surface and the back surface of the single crystal semiconductorhandle substrate, and a bulk region between the front and back surfacesof the single crystal semiconductor handle substrate, wherein the singlecrystal semiconductor handle substrate has a minimum bulk resistivity ofat least about 500 ohm-cm; depositing a dielectric layer on the frontsurface of the single crystal semiconductor handle substrate; andbonding a front surface of a single crystal semiconductor donorsubstrate to the dielectric layer of the single crystal semiconductorhandle substrate to thereby form a bonded structure, wherein the singlecrystal semiconductor donor substrate comprises two major, generallyparallel surfaces, one of which is the front surface of thesemiconductor donor substrate and the other of which is a back surfaceof the semiconductor donor substrate, a circumferential edge joining thefront and back surfaces of the semiconductor donor substrate, and acentral plane between the front and back surfaces of the semiconductordonor substrate, and further wherein the semiconductor donor substratecomprises a cleave plane.

The present invention is still further directed to a method of preparinga multilayer structure, the method comprising: thermally oxidizing afront surface of a single crystal semiconductor handle substrate tothereby grow an oxide layer on the front surface of the single crystalsemiconductor handle substrate, wherein the single crystal semiconductorhandle substrate comprises two major, generally parallel surfaces, oneof which is the front surface of the single crystal semiconductor handlesubstrate and the other of which is a back surface of the single crystalsemiconductor handle substrate, a circumferential edge joining the frontand back surfaces of the single crystal semiconductor handle substrate,a central plane between the front surface and the back surface of thesingle crystal semiconductor handle substrate, and a bulk region betweenthe front and back surfaces of the single crystal semiconductor handlesubstrate, wherein the single crystal semiconductor handle substrate hasa minimum bulk resistivity of at least about 500 ohm-cm; depositing apolycrystalline silicon layer on the oxide layer; depositing adielectric layer on the polycrystalline silicon layer; and bonding afront surface of a single crystal semiconductor donor substrate to thedielectric layer of the single crystal semiconductor handle substrate tothereby form a bonded structure, wherein the single crystalsemiconductor donor substrate comprises two major, generally parallelsurfaces, one of which is the front surface of the semiconductor donorsubstrate and the other of which is a back surface of the semiconductordonor substrate, a circumferential edge joining the front and backsurfaces of the semiconductor donor substrate, and a central planebetween the front and back surfaces of the semiconductor donorsubstrate, and further wherein the semiconductor donor substratecomprises a cleave plane.

The present invention is still further directed to a method of preparinga multilayer structure, the method comprising: epitaxially depositing anepitaxial arsenic-doped silicon layer on a front surface of a singlecrystal semiconductor handle substrate, wherein the single crystalsemiconductor handle substrate comprises two major, generally parallelsurfaces, one of which is the front surface of the single crystalsemiconductor handle substrate and the other of which is a back surfaceof the single crystal semiconductor handle substrate, a circumferentialedge joining the front and back surfaces of the single crystalsemiconductor handle substrate, a central plane between the frontsurface and the back surface of the single crystal semiconductor handlesubstrate, and a bulk region between the front and back surfaces of thesingle crystal semiconductor handle substrate, wherein the singlecrystal semiconductor handle substrate has a minimum bulk resistivity ofat least about 500 ohm-cm; epitaxially depositing an epitaxial siliconlayer on the arsenic-doped silicon layer; depositing a dielectric layeron the silicon layer; and bonding a front surface of a single crystalsemiconductor donor substrate to the dielectric layer of the singlecrystal semiconductor handle substrate to thereby form a bondedstructure, wherein the single crystal semiconductor donor substratecomprises two major, generally parallel surfaces, one of which is thefront surface of the semiconductor donor substrate and the other ofwhich is a back surface of the semiconductor donor substrate, acircumferential edge joining the front and back surfaces of thesemiconductor donor substrate, and a central plane between the front andback surfaces of the semiconductor donor substrate, and further whereinthe semiconductor donor substrate comprises a cleave plane.

Other objects and features will be in part apparent and in part pointedout hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a depiction of an HR-SOI structure comprising a depletionlayer and an n-type inversion region.

FIG. 2 is a depiction of an HR-SOI structure comprising a depletionlayer and a n-type inversion region and further comprising devices inoperation.

FIG. 3 is a depiction of an HR-SOI structure comprising devices inoperation, which depicts an effective bipolar coupling between RFdevices through the depletion layer.

FIG. 4 depicts a single crystal semiconductor handle substrate.

FIGS. 5A and 5B depict a process flow according to some embodiments ofthe present invention.

FIGS. 6A and 6B depict a process flow according to some embodiments ofthe present invention.

FIGS. 7A and 7B depict a process flow according to some embodiments ofthe present invention.

FIGS. 8A, 8B, and 8C depict a process flow according to some embodimentsof the present invention.

FIGS. 9A and 9B depict a SOI structure comprising an isolation regioncomprising a p/n junction.

FIGS. 10A and 10B depict a SOI structure comprising an isolation regioncomprising a dielectric layer.

DETAILED DESCRIPTION OF THE EMBODIMENT(S) OF THE INVENTION

According to the present invention, a method is provided for fabricatinga high resistivity semiconductor on insulator (HR-SOI) wafer forradiofrequency (RF) device applications involving lateral deviceisolation with trenches. The isolation region is extended into thehandle substrate under the dielectric layer, e.g., a buried oxide layer(BOX). Such isolation region impedes the transfer of charge carriersalong the surface of the handle substrate and reduces the parasiticcoupling between RF devices. The present invention proposes to embed anadditional thin isolation region under the surface of the semiconductorhandle substrate. Said isolation region may be based any of p/njunction, dielectric, or semi-insulating material. Said isolation regionfurther suppresses the cross-talk between devices, power losses, andharmonic distortions of device signals.

In general, a high resistivity semiconductor on insulator (HR-SOI) waferfor radiofrequency (RF) device applications is generally p-type (e.g.,boron doped) with a typical resistivity in the range of about 1000ohm-cm to about 2000 ohm-cm. The HR-SOI structure additionally comprisesa dielectric layer, generally a buried oxide layer (BOX) that comprisespositive charges. The typical positive charges in a BOX layer depleteholes and induce n-type inversion at the surface of handle substrates.The width of the depletion region may be between about 5 micrometers andabout 10 micrometers. The inversion layer is several orders on magnitudethinner. See FIG. 1 for a depiction of an HR-SOI structure 2 comprisinga depletion region 6 and n-type inversion layer 8. As shown in FIG. 1,the HR-SOI structure 2 comprises a semiconductor handle substrate 4, aburied oxide layer (BOX) 10, and a semiconductor device layer 12.Positive charges in the buried oxide layer 10 deplete holes in thedepletion region 6 and induce n-type charge inversion 8.

With reference to FIGS. 1 and 2, during a dynamic process of RF device20, 22 operation, at a particular instance, a voltage potential of adevice 20 is altered to negative, while another device 22 is biasedpositively. The n-type inversion layer 8 is a readily available sourceof electrons. Under those devices 20 with negative bias, the n-typeinversion layer 8 injects electrons into the depletion layer 6. Underpositively biased devices 22, an electric field 24 is established suchthat the related n-type inversion layer 8 collects electrons from thedepletion layer 6. Even if the structure 2 comprises an amorphous region26 that separates n-type inversion layer 8 in the semiconductor handlesubstrate 4 under devices 20, 22 to be isolated from each other, aneffective bipolar coupling 30 occurs through the thick depletion layer 6between the devices 20, 22. See FIG. 3.

The present invention provides an SOI structure that eliminates thebipolar coupling between RF devices via trench isolation extend intohandle substrates comprising an amorphous region. This is achieved by athin isolation layer in the handle substrate near its surface. Theisolation layer may be based on any of p/n junction, dielectric, orsemi-insulating material. The depth of the isolation layer should notexceed the depth of the amorphous regions. The isolation layer shouldpreferably be as thin as possible, yet preserving continuity.

I. Semiconductor Handle Substrate and Semiconductor Donor Substrate

The substrates for use in the present invention include a semiconductorhandle substrate, e.g., a single crystal semiconductor handle wafer anda semiconductor donor substrate, e.g., a single crystal semiconductordonor wafer. The semiconductor device layer in asemiconductor-on-insulator composite structure is derived from thesingle crystal semiconductor donor wafer. The semiconductor device layermay be transferred onto the semiconductor handle substrate by waferthinning techniques such as etching a semiconductor donor substrate orby cleaving a semiconductor donor substrate comprising a damage plane.

With reference to FIG. 4, an exemplary, non-limiting single crystalsemiconductor handle wafer 100 is depicted. In general, the singlecrystal semiconductor handle wafer 100 comprises two major, generallyparallel surfaces. One of the parallel surfaces is a front surface 102of the single crystal semiconductor handle wafer 100, and the otherparallel surface is a back surface 104 of the single crystalsemiconductor handle wafer 100. The single crystal semiconductor handlewafer 100 comprises a circumferential edge 106 joining the front andback surfaces 102, 104. The single crystal semiconductor handle wafer100 comprise a central axis 108 perpendicular to the two major,generally parallel surfaces 102, 104 and also perpendicular to a centralplane defined by the points midway between the front and back surfaces102, 104. The single crystal semiconductor handle wafer 100 comprises abulk region 110 between the two major, generally parallel surfaces 102,104. Since semiconductor wafers, e.g., silicon wafers, typically havesome total thickness variation (TTV), warp, and bow, the midpointbetween every point on the front surface 102 and every point on the backsurface 104 may not precisely fall within a plane. As a practicalmatter, however, the TTV, warp, and bow are typically so slight that toa close approximation the midpoints can be said to fall within animaginary central plane which is approximately equidistant between thefront and back surfaces.

Prior to any operation as described herein, the front surface 102 andthe back surface 104 of the single crystal semiconductor handle wafer100 may be substantially identical. A surface is referred to as a “frontsurface” or a “back surface” merely for convenience and generally todistinguish the surface upon which the operations of method of thepresent invention are performed. In the context of the presentinvention, a “front surface” of a single crystal semiconductor handlewafer 100, e.g., a single crystal silicon handle wafer, refers to themajor surface of the substrate that becomes an interior surface of thebonded structure. Accordingly, a “back surface” of a single crystalsemiconductor handle wafer 100, e.g., a handle wafer, refers to themajor surface that becomes an exterior surface of the bonded structure.Similarly, a “front surface” of a single crystal semiconductor donorsubstrate, e.g., a single crystal silicon donor wafer, refers to themajor surface of the single crystal semiconductor donor substrate thatbecomes an interior surface of the bonded structure, and a “backsurface” of a single crystal semiconductor donor substrate, e.g., asingle crystal silicon donor wafer, refers to the major surface thatbecomes an exterior surface of the bonded structure. Upon completion ofconventional bonding and wafer thinning steps, the single crystalsemiconductor donor substrate forms the semiconductor device layer ofthe semiconductor-on-insulator (e.g., silicon-on-insulator) compositestructure.

The single crystal semiconductor handle substrate and the single crystalsemiconductor donor substrate may be single crystal semiconductorwafers. In preferred embodiments, the semiconductor wafers comprise amaterial selected from the group consisting of silicon, silicon carbide,silicon germanium, gallium arsenide, gallium nitride, indium phosphide,indium gallium arsenide, germanium, and combinations thereof. The singlecrystal semiconductor wafers, e.g., the single crystal silicon handlewafer and single crystal silicon donor wafer, of the present inventiontypically have a nominal diameter of at least about 150 mm, at leastabout 200 mm, at least about 300 mm, or at least about 450 mm. Waferthicknesses may vary from about 250 micrometers to about 1500micrometers, such as between about 300 micrometers and about 1000micrometers, suitably within the range of about 500 micrometers to about1000 micrometers. In some specific embodiments, the wafer thickness maybe about 725 micrometers. In some embodiments, the wafer thickness maybe about 775 micrometers.

In particularly preferred embodiments, the single crystal semiconductorwafers comprise single crystal silicon wafers which have been slicedfrom a single crystal ingot grown in accordance with conventionalCzochralski crystal growing methods or float zone growing methods. Suchmethods, as well as standard silicon slicing, lapping, etching, andpolishing techniques are disclosed, for example, in F. Shimura,Semiconductor Silicon Crystal Technology, Academic Press, 1989, andSilicon Chemical Etching, (J. Grabmaier ed.) Springer-Verlag, N.Y., 1982(incorporated herein by reference). Preferably, the wafers are polishedand cleaned by standard methods known to those skilled in the art. See,for example, W. C. O'Mara et al., Handbook of Semiconductor SiliconTechnology, Noyes Publications. If desired, the wafers can be cleaned,for example, in a standard SC1/SC2 solution. In some embodiments, thesingle crystal silicon wafers of the present invention are singlecrystal silicon wafers which have been sliced from a single crystalingot grown in accordance with conventional Czochralski (“Cz”) crystalgrowing methods, typically having a nominal diameter of at least about150 mm, at least about 200 mm, at least about 300 mm, or at least about450 mm. Preferably, both the single crystal silicon handle wafer and thesingle crystal silicon donor wafer have mirror-polished front surfacefinishes that are free from surface defects, such as scratches, largeparticles, etc. Wafer thickness may vary from about 250 micrometers toabout 1500 micrometers, such as between about 300 micrometers and about1000 micrometers, suitably within the range of about 500 micrometers toabout 1000 micrometers. In some specific embodiments, the waferthickness may be about 725 micrometers. In some embodiments, the waferthickness may be about 775 micrometers.

In some embodiments, the single crystal semiconductor wafers, i.e.,single crystal semiconductor handle wafer and single crystalsemiconductor donor wafer, comprise interstitial oxygen inconcentrations that are generally achieved by the Czochralski-growthmethod. In some embodiments, the single crystal semiconductor waferscomprise oxygen in a concentration between about 4 PPMA and about 18PPMA. In some embodiments, the semiconductor wafers comprise oxygen in aconcentration between about 10 PPMA and about 35 PPMA. In someembodiments, the single crystal silicon wafer comprises oxygen in aconcentration of no greater than about 10 PPMA. Interstitial oxygen maybe measured according to SEMI MF 1188-1105.

The single crystal semiconductor handle wafer 100 may have anyresistivity obtainable by the Czochralski or float zone methods.Accordingly, the resistivity of the single crystal semiconductor handlewafer 100 is based on the requirements of the end use/application of thestructure of the present invention. The resistivity may therefore varyfrom milliohm or less to megaohm or more. In some embodiments, thesingle crystal semiconductor handle wafer 100 comprises a p-type or ann-type dopant. Suitable dopants include boron (p type), gallium (ptype), phosphorus (n type), antimony (n type), and arsenic (n type). Thedopant concentration is selected based on the desired resistivity of thehandle wafer. In some embodiments, the single crystal semiconductorhandle substrate comprises a p-type dopant. In some embodiments, thesingle crystal semiconductor handle substrate is a single crystalsilicon wafer comprising a p-type dopant, such as boron.

In some embodiments, the single crystal semiconductor handle wafer 100has a relatively low minimum bulk resistivity, such as below about 100ohm-cm, below about 50 ohm-cm, below about 1 ohm-cm, below about 0.1ohm-cm, or even below about 0.01 ohm-cm. In some embodiments, the singlecrystal semiconductor handle substrate 100 has a relatively low minimumbulk resistivity, such as below about 100 ohm-cm, or between about 1ohm-cm and about 100 ohm-cm. Low resistivity wafers may compriseelectrically active dopants, such as boron (p type), gallium (p type),aluminum (p type), indium (p type), phosphorus (n type), antimony (ntype), and arsenic (n type).

In some embodiments, the single crystal semiconductor handle wafer 100has a relatively high minimum bulk resistivity. High resistivity wafersare generally sliced from single crystal ingots grown by the Czochralskimethod or float zone method. High resistivity wafers may compriseelectrically active dopants, such as boron (p type), gallium (p type),aluminum (p type), indium (p type), phosphorus (n type), antimony (ntype), and arsenic (n type), in generally very low concentrations.Cz-grown silicon wafers may be subjected to a thermal anneal at atemperature ranging from about 600° C. to about 1000° C. in order toannihilate thermal donors caused by oxygen that are incorporated duringcrystal growth. In some embodiments, the single crystal semiconductorhandle wafer has a minimum bulk resistivity of at least 100 Ohm-cm, oreven at least about 500 Ohm-cm, such as between about 100 Ohm-cm andabout 100,000 Ohm-cm, or between about 500 Ohm-cm and about 100,000Ohm-cm, or between about 1000 Ohm-cm and about 100,000 Ohm-cm, orbetween about 500 Ohm-cm and about 10,000 Ohm-cm, or between about 750Ohm-cm and about 10,000 Ohm-cm, between about 1000 Ohm-cm and about10,000 Ohm-cm, between about 1000 Ohm-cm and about 6000 ohm-cm, betweenabout 2000 Ohm-cm and about 10,000 Ohm-cm, between about 3000 Ohm-cm andabout 10,000 Ohm-cm, or between about 3000 Ohm-cm and about 5,000Ohm-cm. In some preferred embodiments, the single crystal semiconductorhandle substrate has a bulk resistivity between about 1000 Ohm-cm andabout 6,000 Ohm-cm. In some preferred embodiments, the single crystalsemiconductor handle substrate comprises an electrically active dopantselected from the group consisting of boron, aluminum, gallium, indium,and any combination thereof. In some preferred embodiments, the singlecrystal semiconductor handle substrate comprises boron, which may bepresent in a concentration less than about 2×10¹³ atoms/cm³, less thanabout 1×10¹³ atoms/cm³, such as less than about 5×10¹² atoms/cm³, orless than about 1×10¹² atoms/cm³. Methods for preparing high resistivitywafers are known in the art, and such high resistivity wafers may beobtained from commercial suppliers, such as SunEdison Semiconductor Ltd.(St. Peters, Mo.; formerly MEMC Electronic Materials, Inc.).

The single crystal semiconductor handle wafer 100 may comprise singlecrystal silicon. The single crystal semiconductor handle wafer 100 mayhave any of (100), (110), or (111) crystal orientation, and the choiceof crystal orientation may be dictated by the end use of the structure.

Optionally, the front surface 102, the back surface 104, or both may beoxidized according to methods known in the art. Oxidation may beaccomplished by means known in the art, such as thermal oxidation (inwhich some portion of the deposited semiconductor material film will beconsumed), CVD oxide deposition, or by exposure to a standard cleaningsolution, such as an SC1/SC2 cleaning solution. The oxidation layer onthe front surface 102, the back surface 104, or both may be may be atleast about 1 nanometer thick, such as between about 10 nanometers andabout 5000 nanometers thick, such as between about 100 nanometers andabout 1000 nanometers, or between about 200 nanometers and about 400nanometers. In some embodiments, the oxidation layer is relatively thin,such as between about 5 angstroms and about 25 angstroms, such asbetween about 10 angstroms and about 15 angstroms. Thin oxide layers canbe obtained by exposure to a standard cleaning solution, such as anSC1/SC2 cleaning solution.

II. Forming an Isolation Region in the Handle Substrate by Arsenic IonImplantation or by Oxygen Ion Implantation

According to some embodiments of the method of the present invention, anisolation region in a single crystal semiconductor handle wafer may beformed by ion implantation. Suitable ions for implantation may includearsenic, e.g., As⁺, to form an arsenic-doped N+ layer (p/n junction) oroxygen, e.g., O⁺ and/or O₂ ⁺, to form a dielectric or semi-insulatingsemiconductor oxide layer buried in the handle wafer. In someembodiments, both arsenic, e.g., As⁺, and oxygen, e.g., O⁺ or O₂ ⁺, areimplanted. See FIG. 5A, which depicts ion implantation 120, either As⁺ion implantation or O⁺ ion implantation or both, through a front surface102 of a single crystal semiconductor handle wafer 100. FIG. 5B depictsa single crystal semiconductor handle wafer 100 comprising an isolationregion 120 formed by either As⁺ ion implantation or O⁺ ion implantationor both through a front surface 102 of a single crystal semiconductorhandle wafer 100.

In some embodiments, arsenic ions, As⁺, are implanted to form anarsenic-doped N+ layer (p/n junction) as the isolation region. Theisolation region 120 therefore comprises arsenic-doped semiconductor. Insome embodiments, the single crystal semiconductor handle wafer 100comprises a single crystal silicon handle wafer, and the isolationregion 120 therefore comprises arsenic-doped silicon. Ion implantationmay be carried out in a commercially available instrument, such as anApplied Materials Quantum II, a Quantum LEAP, or a Quantum X. Arsenicions, As⁺, may be implanted at a dosage between about 1×10¹² atoms/cm²to about 5×10¹⁴ atoms/cm², such as between about 5×10¹² atoms/cm² toabout 1×10¹⁴ atoms/cm². Arsenic ion implant energies may range fromabout 1 keV to about 500 keV, such as from about 10 keV to about 100keV, such as between about 10 keV and about 50 keV, such as about 20keV. In some embodiments, the As⁺ ions are implanted to a depth ofbetween about 20 angstroms and about 400 angstroms, such as betweenabout 50 angstroms and about 200 angstroms, as measured from the frontsurface of the single crystal semiconductor handle substrate toward thecentral plane. The isolation region 120 should be thinner than the depthof the amorphous region (see FIGS. 9A, 9B, 10A, 10B) so the thickness ofthe isolation region 120 depends in part upon the configuration of thedevice, in which the amorphous region may vary in thickness. In someembodiments, the isolation region 120 may be between about 50 angstromsand about 10,000 angstroms thick, or between about 200 angstroms andabout 10,000 angstroms thick, such as between about 300 angstroms andabout 1000 angstroms, or between about 50 angstroms and about 200angstroms. In some embodiments, the isolation region may have aresistivity between about 0.25×10⁶ ohm-cm and about 1×10¹⁴ ohm-cm, suchas between about 1×10¹² ohm-cm and about 1×10¹⁴ ohm-cm.

In some embodiments, oxygen ions, e.g., O⁺ and/or O₂ ⁺, are implanted toform a semi-insulating or dielectric layer as the isolation region. Theisolation region 120 therefore comprises semiconductor oxide. In someembodiments, the single crystal semiconductor handle wafer 100 comprisesa single crystal silicon handle wafer, and the isolation region 120therefore comprises silicon dioxide. Ion implantation may be carried outin a commercially available instrument, such as an Applied MaterialsQuantum II, a Quantum LEAP, or a Quantum X. Oxygen ions, O⁺ and/or O₂ ⁺,may be implanted at a dosage between about 5×10¹⁴ atoms/cm² to about5×10¹⁷ atoms/cm², such as between about 1×10¹⁵ atoms/cm² to about 1×10¹⁷atoms/cm². Oxygen ion implant energies may range from about 1 keV toabout 200 keV, such as from about 5 keV to about 50 keV, such as betweenabout 5 keV and about 20 keV, such as about 10 keV. In some embodiments,the O⁺ and/or O₂ ⁺ ions are implanted to a depth of between about 20angstroms and about 400 angstroms, such as between about 50 angstromsand about 200 angstroms, as measured from the front surface of thesingle crystal semiconductor handle substrate toward the central plane.The isolation region 120 should be thinner than the depth of theamorphous region (see FIGS. 9A, 9B, 10A, 10B) so the thickness of theisolation region 120 depends in part upon the configuration of thedevice, in which the amorphous region may vary in thickness. In someembodiments, the isolation region 120 may be between about 50 angstromsand about 10,000 angstroms thick, or between about 200 angstroms andabout 10,000 angstroms thick, such as between about 300 angstroms andabout 1000 angstroms, or between about 50 angstroms and about 200angstroms. In some embodiments, the isolation region may have aresistivity between about 0.25×10⁶ ohm-cm and about 1×10¹⁴ ohm-cm, suchas between about 1×10¹² ohm-cm and about 1×10¹⁴ ohm-cm.

In some embodiments, the ion implantation method advantageously disruptsthe crystallinity of the front surface region of the handle substrate,the front surface region being defined as the region between the frontsurface 102 of the handle substrate 100 and the isolation layer 120. SeeFIG. 5B. Accordingly, the front surface region of the handle substratemay comprise polycrystalline semiconductor or amorphous semiconductor.Polycrystalline semiconductor, e.g., polycrystalline silicon, denotes amaterial comprising small silicon crystals having random crystalorientations. Polycrystalline semiconductor, e.g., polycrystallinesilicon, grains may be as small in size as about 20 nanometers. Thepolycrystalline semiconductor, e.g., polycrystalline silicon, grains maybe as large as 1000 nanometers. In some embodiments, the polycrystallinesemiconductor, e.g., polycrystalline silicon, grains may have an averagesize between about 20 nanometers and about 500 nanometers. Amorphoussemiconductor, e.g., amorphous silicon, comprises a non-crystallineallotropic form of silicon, which lacks short range and long rangeorder. Semiconductor, e.g., silicon, grains having crystallinity over nomore than about 10 nanometers may also be considered essentiallyamorphous.

III. Forming an Isolation Region in the Handle Substrate by ThermalOxide Film Deposition

According to some embodiments of the method of the present invention, anisolation region in a single crystal semiconductor handle wafer may beformed by growth of an oxide region, following by growth a semiconductormaterial. See FIG. 6A and FIG. 6B for a depiction of the growth of anisolation region 200 comprising semiconductor oxide by oxidation of thesingle crystal semiconductor handle wafer 100. Oxidation of the singlecrystal semiconductor handle wafer 100 may be accomplished by meansknown in the art, such as thermal oxidation (in which some portion ofthe deposited semiconductor material film will be consumed) or CVD oxidedeposition.

In general, a thermal oxidation method oxidizes both the front surfaceand the back surface of a single crystal semiconductor handle wafer 100or single crystal semiconductor donor wafer, unless a masking techniqueis employed to inhibit oxidation of a side, or a portion of a side, of awafer. In some embodiments, oxidation of both the front side and theback side of a wafer is advantageous to offset compressive stresses thatmay otherwise result in wafer bow for a wafer having an oxidation layeron only one side of the wafer. In some embodiments, the single crystalsemiconductor handle substrate 100, e.g., a single crystal siliconhandle wafer, may be thermally oxidized in a furnace such as an ASM A400or ASM A400XT. The temperature may range from 750° C. to 1200° C. in anoxidizing ambient. The oxidizing ambient atmosphere can be a mixture ofinert gas, such as Ar or N₂, and O₂. The oxygen content may vary from 1to 10 percent, or higher. In some embodiments, the oxidizing ambientatmosphere may be up to 100% (a “dry oxidation”). In some embodiments,the ambient atmosphere may comprise a mixture of inert gas, such as Aror N₂, and oxidizing gases, such as O₂ and water vapor (a “wetoxidation”). Optionally, the ambient atmosphere may contain hydrochloricacid, e.g., up to about 10 volume %, to remove surface impurities duringoxidation. In an exemplary embodiment, single crystal semiconductorhandle wafer 100 may be loaded into a vertical furnace, such as an ASMA400 or ASM A400XT. The temperature is ramped to the oxidizingtemperature with a mixture of N₂ and O₂. After the desired oxidethickness has been obtained, the O₂ is turned off and the furnacetemperature is reduced and wafers are unloaded from the furnace. Theoxidation layer 200 may be between about 20 angstroms and about 400angstroms thick, such as between 50 angstroms and about 200 angstroms.In thermal oxidation methods in which both the front and back surfacesare oxidized, the silicon dioxide layers are generally the samethicknesses front and back, although techniques may be used, such asetching or polishing, to vary the thicknesses.

In some embodiments, the front surface, the back surface, or both thefront and back surfaces of the single crystal semiconductor handle wafer100 may be oxidized by a chemical vapor deposition process, such asplasma enhanced chemical vapor deposition or low pressure chemical vapordeposition. The oxidation layer 200 may be between about 20 angstromsand about 400 angstroms thick, such as between 50 angstroms and about200 angstroms.

Plasma enhanced chemical vapor deposition may occur in a commerciallyavailable tool, such as those available from EV Group, such as EVG®810LTLow Temp Plasma Activation System. General requirements of a plasmaenhanced CVD chamber include a reactor with various electrode designs,power generation electronics, impedance matching network to transferpower to the gas load, mass flow controllers for input gasses, andpressure control systems. Typical systems are vertical tube reactorspowered by an inductively coupled RF source. The single crystalsemiconductor handle wafer 100 and/or donor wafer is loaded into thechamber and placed on a heated support chamber. The chamber is evacuatedand backfilled with an oxygen gas source in a carrier gas, such asargon, to a pressure less than atmospheric to thereby create the plasma.Oxygen and/or water are suitable source gases for plasma oxidetreatment. To deposit a silicon nitride or a silicon dioxide plasmafilm, suitable silicon precursors include methyl silane, silicontetrahydride (silane), trisilane, disilane, pentasilane, neopentasilane,tetrasilane, dichlorosilane (SiH₂Cl₂), trichlorosilane (SiHCl₃), silicontetrachloride (SiCl₄), tetra-ethyl orthosilicate (Si(OCH₂CH₃)₄), amongothers. Plasma deposition may occur at a temperature between about 20°C. and about 400° C. The flow rate ratios of the gaseous siliconprecursor and the gaseous oxygen precursor may be between about 1/200and about 1/50, such as about 1/100.

The LPCVD process can be done in a cold or hot walled quartz tubereactor. Hot walled furnaces allow batch processing and therefore highthroughput. They also provide good thermal uniformity, and thus resultin uniform films. A disadvantage of hot wall systems is that depositionalso occurs on the furnace walls, so that frequent cleaning orreplacement of the tube is necessary to avoid flaking of the depositedmaterial and subsequent particle contamination. Cold wall reactors arelower maintenance, as there is no film deposition on the reactor walls.The low pressure chemical vapor semiconductor oxide (e.g., silicondioxide) may be formed at pressures between about 0.01 Torr and about100 Torr, such as between about 0.1 Torr and about 1 Torr in a lowpressure chemical vapor deposition. Temperatures may range between 425°C. and 900° C. Suitable precursors include those listed for PECVD.

According to the method of the present invention and with reference toFIG. 6B, a polycrystalline or amorphous semiconductor material 202 isdeposited onto the oxidation layer 200 of the single crystalsemiconductor handle wafer 100. Such materials include polycrystallinesemiconductor materials and amorphous semiconductor materials. Materialsthat may be polycrystalline or amorphous include silicon (Si), silicongermanium (SiGe), silicon doped with carbon (SiC), and germanium (Ge).Polycrystalline silicon denotes a material comprising small siliconcrystals having random crystal orientations. Polycrystalline silicongrains may be as small in size as about 20 nanometers. Thepolycrystalline silicon grains may be as large as 1000 nanometers. Insome embodiments, the polycrystalline silicon grains may have an averagesize between about 20 nanometers and about 500 nanometers. According tothe method of the present invention, the smaller the crystal grain sizeof polycrystalline silicon deposited the higher the defectivity in thecharge trapping layer. Amorphous silicon comprises a non-crystallineallotropic form of silicon, which lacks short range and long rangeorder. Silicon grains having crystallinity over no more than about 10nanometers may also be considered essentially amorphous.

The material for deposition onto the oxidation layer 200 of the singlecrystal semiconductor handle wafer 100 may be deposited by means knownin the art. For example, the semiconductor material may be depositedusing metalorganic chemical vapor deposition (MOCVD), physical vapordeposition (PVD), chemical vapor deposition (CVD), low pressure chemicalvapor deposition (LPCVD), plasma enhanced chemical vapor deposition(PECVD), or molecular beam epitaxy (MBE). Silicon precursors for LPCVDor PECVD include methyl silane, silicon tetrahydride (silane),trisilane, disilane, pentasilane, neopentasilane, tetrasilane,dichlorosilane (SiH₂Cl₂), silicon tetrachloride (SiCl₄), among others.For example, polycrystalline silicon may be deposited onto the oxidationlayer 200 of the single crystal semiconductor handle wafer 100 bypyrolyzing silane (SiH₄) in a temperature range between about 550° C.and about 690° C., such as between about 580° C. and about 650° C. Thechamber pressure may range from about 70 to about 400 mTorr. Amorphoussilicon may be deposited by plasma enhanced chemical vapor deposition(PECVD) at temperatures generally ranging between about 75° C. and about300° C. The overall thickness of the polycrystalline or amorphoussemiconductor material 202 may be between about 50 nanometers and about4000 nanometers, such as between about 100 nanometers and about 2000nanometers.

V. Forming an Isolation Region in the Handle Substrate by EpitaxialDeposition of Arsenic Doped Layer

According to some embodiments of the method of the present invention, anisolation region in a single crystal semiconductor handle wafer 100 maybe formed by epitaxial deposition of an arsenic-doped silicon layer,following by epitaxial deposition of a semiconductor material. See FIG.7A and FIG. 7B for a depiction of the growth of an isolation region 300comprising arsenic-doped silicon by epitaxial deposition of such a layeron the front surface of the single crystal semiconductor handle wafer100. Epitaxially deposited material matches the crystalline order of theunderlying substrate. Epitaxial deposition may occur by chemical vapordeposition (CVD), low pressure chemical vapor deposition (LPCVD), plasmaenhanced chemical vapor deposition (PECVD), or molecular beam epitaxy(MBE). Epitaxial deposition may occur by vapor phase or liquid phasedeposition. Silicon precursors for epitaxial deposition include methylsilane, silicon tetrahydride (silane), trisilane, disilane, pentasilane,neopentasilane, tetrasilane, dichlorosilane (SiH₂Cl₂), silicontetrachloride (SiCl₄), among others. Arsenic precursors for epitaxialdeposition include alkyl arsines, such as mono 1-methylethylarsine, mono1-methylpropylarsine, mono 1,1-dimethylethylarsine, mono1-methylbutylarsine, mono 1,1-dimethylpropylarsine, mono1,1-dimethylbutylarsine, mono 1,1-diethylethylarsine, mono1,1,2-trimethylpropylarsine, mono 1,1,3,3-tetramethylbutylarsine, mono1-methylheptylarsine, mono 1-methylnonylarsine, monoα-methylbenzylarsine, mono α, α-dimethylbenzylarsine, monoα-methyl(4-methylbenzyl)arsine, bis(1-methylethyl)arsine,bis(1-methylpropyl)arsine, bis(1,1-dimethylethyl)arsine,bis(1-methylbutyl)arsine, bis(1,1-dimethylpropyl)arsine,bis(1,1-dimethylbutyl)arsine, bis(1,1,3,3,-tetramethylbutyl)arsine,bis(α-methylbenzyl) arsine and the like. The isolation region 300comprising arsenic doped silicon may be between about 20 angstroms andabout 400 angstroms thick, such as between 50 angstroms and about 200angstroms. In some embodiments, the isolation region may have aresistivity between about 0.25×10⁶ ohm-cm and about 1×10¹⁴ ohm-cm, suchas between about 1×10¹² ohm-cm and about 1×10¹⁴ ohm-cm.

Following deposition of the isolation region 300 comprising arsenicdoped silicon, the flow of the arsenic precursor is stopped, andepitaxial deposition continues with only the silicon precursor tothereby deposit an epitaxial silicon layer. With reference to FIG. 7B,epitaxial deposition continues until an epitaxial silicon layer 302 isdeposited having a thickness between about 50 nanometers and about 4000nanometers, such as between about 100 nanometers and about 2000nanometers.

VI. Deposition of Semiconductor Oxide on Handle Substrate

In some embodiments and with reference to FIGS. 8A and 8B, a dielectriclayer 500 is deposited on the front surface 102 of the single crystalsemiconductor handle substrate 100. The dielectric layer 500 maycomprise a material selected from the group consisting of silicondioxide, silicon oxynitride, silicon nitride, hafnium oxide, titaniumoxide, zirconium oxide, lanthanum oxide, barium oxide, and a combinationthereof. In some embodiments, the dielectric layer 500 comprises aburied oxide layer having a thickness of at least about 10 nanometerthick, such as between about 10 nanometers and about 10,000 nanometers,between about 10 nanometers and about 5,000 nanometers, or between about100 nanometers and about 400 nanometers.

In some embodiments and with reference to FIGS. 8A and 8B, the frontsurface of the single crystal semiconductor handle substrate 100 (e.g.,a silicon wafer) comprising the isolation region 400 is oxidized to forma dielectric layer 500 (a semiconductor oxide layer, e.g., a silicondioxide layer) on the front surface 102 of the single crystalsemiconductor handle substrate 100. The isolation region 400 is preparedaccording to one of the methods set forth herein above. The dielectriclayer 500 (a semiconductor oxide layer, e.g., a silicon dioxide layer)is suitable as a buried oxide layer in the manufacture of asemiconductor-on-insulator structure (e.g., silicon-on-insulator)comprising a single crystal semiconductor handle substrate 100comprising an isolation region 400, a semiconductor oxide (e.g., silicondioxide) layer 500, and a single crystal semiconductor device layer. Inorder to form a semiconductor oxide layer 500 (e.g., a silicon dioxidelayer), the front surface of the single crystal semiconductor handlesubstrate 100 (e.g., a silicon wafer) is oxidized. This may beaccomplished by means known in the art, such as thermal oxidation (inwhich some portion of the deposited semiconductor material film will beconsumed) or CVD oxide deposition.

In some embodiments, the single crystal semiconductor handle substrate100, e.g., a single crystal silicon handle wafer, comprising theisolation region 400 may be thermally oxidized in a furnace such as anASM A400 or ASM A400XT. The temperature may range from 750° C. to 1200°C. in an oxidizing ambient. The oxidizing ambient atmosphere can be amixture of inert gas, such as Ar or N₂, and O₂. The oxygen content mayvary from 1 to 10 percent, or higher. In some embodiments, the oxidizingambient atmosphere may be up to 100% (a “dry oxidation”). In someembodiments, the ambient atmosphere may comprise a mixture of inert gas,such as Ar or N₂, and oxidizing gases, such as O₂ and water vapor (a“wet oxidation”). Optionally, the ambient atmosphere may containhydrochloric acid, e.g., up to about 10 volume %, to remove surfaceimpurities during oxidation. In an exemplary embodiment, the singlecrystal semiconductor handle substrate 100 may be loaded into a verticalfurnace, such as an ASM A400 or ASM A400XT. The temperature is ramped tothe oxidizing temperature with a mixture of N₂ and O₂. After the desiredoxide thickness has been obtained, the O₂ is turned off and the furnacetemperature is reduced and wafers are unloaded from the furnace. In someembodiments, the semiconductor oxide layer 500 (e.g., a silicon dioxidelayer), is formed on the front surface of the single crystalsemiconductor handle substrate 100 to an average depth D1 between about10 nanometers and about 10,000 nanometers, between about 10 nanometersand about 5,000 nanometers, or between about 100 nanometers and about400 nanometers.

In some embodiments, the semiconductor oxide layer 500 may be depositedonto the single crystal semiconductor handle substrate 100, e.g., asingle crystal silicon handle wafer, comprising the isolation region 400by CVD. Suitable techniques include metalorganic chemical vapordeposition (MOCVD), physical vapor deposition (PVD), chemical vapordeposition (CVD), low pressure chemical vapor deposition (LPCVD), plasmaenhanced chemical vapor deposition (PECVD), or molecular beam epitaxy(MBE). Silicon precursors for LPCVD or PECVD include methyl silane,silicon tetrahydride (silane), trisilane, disilane, pentasilane,neopentasilane, tetrasilane, dichlorosilane (SiH₂Cl₂), silicontetrachloride (SiCl₄), among others. Oxygen may be provided by oxygen orozone. Silicon oxide may be deposited onto the front surface of thesingle crystal semiconductor handle substrate 100 by pyrolyzing silane(SiH₄) and oxygen in a temperature range between about 550° C. and about690° C., such as between about 580° C. and about 650° C. The chamberpressure may range from about 70 to about 400 mTorr. In someembodiments, the semiconductor oxide layer 500 (e.g., a silicon dioxidelayer), is formed by CVD on the front surface of the single crystalsemiconductor handle substrate 100 to an average depth D1 between about10 nanometers and about 10,000 nanometers, between about 10 nanometersand about 5,000 nanometers, or between about 100 nanometers and about400 nanometers.

After oxidation, wafer cleaning is optional. If desired, the wafer canbe cleaned, for example, in a standard SC1/SC2 solution.

VII. Wafer Bonding

With reference to FIGS. 8A, 8B, and 8C, the single crystal semiconductorhandle substrate 100, e.g. a single crystal semiconductor handle wafersuch as a single crystal silicon handle wafer, prepared according to themethod described herein comprising an isolation region 400 and adielectric layer 500 is next bonded to a semiconductor donor substrate600, e.g., a single crystal semiconductor donor wafer, which is preparedaccording to conventional layer transfer methods. The single crystalsemiconductor donor substrate 600 may be single crystal semiconductorwafers. In preferred embodiments, the semiconductor wafers comprise amaterial selected from the group consisting of silicon, silicon carbide,silicon germanium, gallium arsenide, gallium nitride, indium phosphide,indium gallium arsenide, germanium, and combinations thereof. Dependingupon the desired properties of the final integrated circuit device, thesingle crystal semiconductor (e.g., silicon) donor wafer 600 maycomprise electrically active dopants, such as boron (p type), gallium (ptype), aluminum (p type), indium (p type), phosphorus (n type), antimony(n type), and arsenic (n type). In some embodiments, the donor wafer maycomprise a combination of p type dopants. In some embodiments, the donorwafer may comprise a combination of n type dopants. In some embodiments,the donor wafer may comprise a combination of p type dopants and n typedopants. The resistivity of the single crystal semiconductor (e.g.,silicon) donor wafer may range from 1 to 50 Ohm-cm, typically, from 5 to25 Ohm-cm. The single crystal semiconductor donor wafer may be subjectedto standard process steps including oxidation, implant, and post implantcleaning. Accordingly, a single crystal semiconductor donor substrate600 (e.g., a single crystal semiconductor donor wafer), such as a singlecrystal semiconductor wafer of a material that is conventionally used inpreparation of multilayer semiconductor structures, e.g., a singlecrystal silicon donor wafer, that has been etched and polished andoptionally oxidized, is subjected to ion implantation to form a damagelayer in the donor substrate.

In some embodiments, the single crystal semiconductor donor substrate600 comprises a dielectric layer. The dielectric layer may comprise oneor more insulating layers formed on the front surface of the singlecrystal semiconductor donor wafer 600. The dielectric layer according tothe present invention may comprise insulating materials selected fromamong silicon dioxide, silicon nitride, silicon oxynitride, hafniumoxide, titanium oxide, zirconium oxide, lanthanum oxide, barium oxide,and any combination thereof. In some embodiments, the dielectric layercomprises one or more insulating layers comprising a material selectedfrom the group consisting of silicon dioxide, silicon nitride, siliconoxynitride, and any combination thereof. In some embodiments, thedielectric layer has a thickness of at least about 10 nanometer thick,such as between about 10 nanometers and about 10,000 nanometers, betweenabout 10 nanometers and about 5,000 nanometers, between 50 nanometersand about 400 nanometers, or between about 100 nanometers and about 400nanometers, such as about 50 nanometers, 100 nanometers, or 200nanometers.

In some embodiments, the dielectric layer comprises multiple layers ofinsulating material. The dielectric layer may comprise two insulatinglayers, three insulating layers, or more. Each insulating layer maycomprise a material selected from among silicon dioxide, siliconoxynitride, silicon nitride, hafnium oxide, titanium oxide, zirconiumoxide, lanthanum oxide, barium oxide, and any combination thereof. Insome embodiments, each insulating layer may comprise a material selectedfrom the group consisting of silicon dioxide, silicon nitride,siliconoxynitride, and any combination thereof. Each insulating layermay have a thickness of at least about 10 nanometer thick, such asbetween about 10 nanometers and about 10,000 nanometers, between about10 nanometers and about 5,000 nanometers, between 50 nanometers andabout 400 nanometers, or between about 100 nanometers and about 400nanometers, such as about 50 nanometers, 100 nanometers, or 200nanometers.

In some embodiments, the front surface of the single crystalsemiconductor donor substrate 600 (e.g., a single crystal silicon donorsubstrate) may be thermally oxidized (in which some portion of thedeposited semiconductor material film will be consumed) to prepare thesemiconductor oxide film, or the semiconductor oxide (e.g., silicondioxide) film may be grown by CVD oxide deposition. In some embodiments,the front surface of the single crystal semiconductor donor substrate600 may be thermally oxidized in a furnace such as an ASM A400 or ASMA400XT in the same manner described above. In some embodiments, thedonor substrates are oxidized to provide an oxide layer on the frontsurface layer of at least about 1 nanometer thick, such as between about10 nanometers and about 5000 nanometers thick, such as between about 100nanometers and about 1000 nanometers, or between about 200 nanometersand about 400 nanometers. In some embodiments, the oxidation layer isrelatively thin, such as between about 5 angstroms and about 25angstroms, such as between about 10 angstroms and about 15 angstroms.

Ion implantation may be carried out in a commercially availableinstrument, such as an Applied Materials Quantum II, a Quantum LEAP, ora Quantum X. Implanted ions include He, H, H₂, or combinations thereof.Ion implantation is carried out as a density and duration sufficient toform a damage layer in the semiconductor donor substrate. Implantdensity may range from about 10¹² ions/cm² to about 10¹⁷ ions/cm², suchas from about 10¹⁴ ions/cm² to about 10¹⁷ ions/cm², such as from about10¹⁵ ions/cm² to about 10¹⁶ ions/cm². Implant energies may range fromabout 1 keV to about 3,000 keV, such as from about 5 keV to about 1,000keV, or from about 5 keV to about 200 keV, or from 5 keV to about 100keV, or from 5 keV to about 80 keV. The depth of implantation determinesthe thickness of the single crystal semiconductor device layer in thefinal SOI structure. The ions may be implanted to a depth between about100 angstroms and about 30,000 angstroms, such as between about 200angstroms and about 20,000 angstroms, such as between about 2000angstroms and about 15,000 angstroms, or between about 15,000 angstromsand about 30,000 angstroms. In some embodiments it may be desirable tosubject the single crystal semiconductor donor wafers, e.g., singlecrystal silicon donor wafers, to a clean after the implant. In somepreferred embodiments, the clean could include a Piranha clean followedby a DI water rinse and SC1/SC2 cleans.

In some embodiments of the present invention, the single crystalsemiconductor donor substrate 600 having an ion implant region thereinformed by He⁺, H⁺, H₂ ⁺, and any combination thereof ion implant isannealed at a temperature sufficient to form a thermally activatedcleave plane in the single crystal semiconductor donor substrate. Anexample of a suitable tool might be a simple Box furnace, such as a BlueM model. In some preferred embodiments, the ion implanted single crystalsemiconductor donor substrate is annealed at a temperature of from about200° C. to about 350° C., from about 225° C. to about 350° C.,preferably about 350° C. Thermal annealing may occur for a duration offrom about 2 hours to about 10 hours, such as from about 2 hours toabout 8 hours. Thermal annealing within these temperatures ranges issufficient to form a thermally activated cleave plane. After the thermalanneal to activate the cleave plane, the single crystal semiconductordonor substrate surface is preferably cleaned.

In some embodiments, the ion-implanted and optionally cleaned andoptionally annealed single crystal semiconductor donor substrate issubjected to oxygen plasma and/or nitrogen plasma surface activation. Insome embodiments, the oxygen plasma surface activation tool is acommercially available tool, such as those available from EV Group, suchas EVG®810LT Low Temp Plasma Activation System. The ion-implanted andoptionally cleaned single crystal semiconductor donor wafer is loadedinto the chamber. The chamber is evacuated and backfilled with O₂ or N₂to a pressure less than atmospheric to thereby create the plasma. Thesingle crystal semiconductor donor wafer is exposed to this plasma forthe desired time, which may range from about 1 second to about 120seconds. Oxygen or nitrogen plasma surface oxidation is performed inorder to render the front surface of the single crystal semiconductordonor substrate hydrophilic and amenable to bonding to a single crystalsemiconductor handle substrate prepared according to the methoddescribed above. After plasma activation, the activated surface isrinsed with deionized water. The wafer is then spun dry prior tobonding.

The hydrophilic front surface layer of the single crystal semiconductordonor substrate 600 and the dielectric layer 500 in contact with thesingle crystal semiconductor handle substrate 100 are next brought intointimate contact to thereby form a bonded structure. See FIG. 8B Thebonded structure comprises a dielectric layer 500, e.g., a buried oxide,between the single crystal semiconductor handle substrate 100 comprisingthe isolation region 400 and the single crystal semiconductor donorsubstrate 600. A portion of the dielectric layer 500 is contributed bythe oxidized front surface of the single crystal semiconductor handlesubstrate 100 and, optionally, a portion of the dielectric layercontributed by the oxidized front surface of the single crystalsemiconductor donor substrate 600. In some embodiments, the dielectriclayer 500 comprises a buried oxide layer having a thickness of at leastabout 10 nanometer thick, such as between about 10 nanometers and about10,000 nanometers, between about 10 nanometers and about 5,000nanometers, or between about 100 nanometers and about 400 nanometers.

Since the mechanical bond is relatively weak, the bonded structure maybe further annealed to solidify the bond between the single crystalsemiconductor donor substrate 600 and the single crystal semiconductorhandle substrate 100. In some embodiments of the present invention, thebonded structure is annealed at a temperature sufficient to form athermally activated cleave plane in the single crystal semiconductordonor substrate. An example of a suitable tool might be a simple Boxfurnace, such as a Blue M model. In some preferred embodiments, thebonded structure is annealed at a temperature of from about 200° C. toabout 350° C., from about 225° C. to about 350° C., preferably about350° C. Thermal annealing may occur for a duration of from about 0.5hours to about 10 hour, preferably a duration of about 2 hours. Thermalannealing within these temperatures ranges is sufficient to form athermally activated cleave plane. After the thermal anneal to activatethe cleave plane, the bonded structure may be cleaved.

After the thermal anneal, the bond between the single crystalsemiconductor donor substrate 600 and the single crystal semiconductorhandle substrate 100 is strong enough to initiate layer transfer viacleaving the bonded structure at the cleave plane. Cleaving may occuraccording to techniques known in the art. In some embodiments, thebonded structure may be placed in a conventional cleave station affixedto stationary suction cups on one side and affixed by additional suctioncups on a hinged arm on the other side. A crack is initiated near thesuction cup attachment and the movable arm pivots about the hingecleaving the wafer apart. Cleaving removes a portion of thesemiconductor donor wafer, thereby leaving a single crystalsemiconductor device layer 602, preferably a silicon device layer, onthe semiconductor-on-insulator composite structure. See FIG. 8C.

After cleaving, the cleaved structure may be subjected to a hightemperature anneal in order to further strengthen the bond between thetransferred device layer 602 and the single crystal semiconductor handlesubstrate 100. An example of a suitable tool might be a verticalfurnace, such as an ASM A400 or ASM A400XT. In some preferredembodiments, the bonded structure is annealed at a temperature of fromabout 1000° C. to about 1200° C., preferably at about 1000° C. Thermalannealing may occur for a duration of from about 0.5 hours to about 8hours, preferably a duration of about 4 hours. Thermal annealing withinthese temperatures ranges is sufficient to strengthen the bond betweenthe transferred device layer and the single crystal semiconductor handlesubstrate.

After the cleave and high temperature anneal, the bonded structure maybe subjected to a cleaning process designed to remove thin thermal oxideand clean particulates from the surface. In some embodiments, the singlecrystal semiconductor device layer 602 may be brought to the desiredthickness and smoothness by subjecting to a vapor phase HCl etch processin a horizontal flow single wafer epitaxial reactor using H₂ as acarrier gas. In some embodiments, the semiconductor device layer 602 mayhave a thickness between about 10 nanometers and about 20 micrometers,between about 20 nanometers and about 3 micrometers, such as betweenabout 20 nanometers and about 2 micrometers, such as between about 20nanometers and about 1.5 micrometers or between about 1.5 micrometersand about 3 micrometers. Thick film device layers may have a devicelayer thickness between about 1.5 micrometers and about 20 micrometers.Thin film device layers may have a thickness between about 0.01micrometer and about 0.20 micrometer.

In some embodiments, an epitaxial layer may be deposited on thetransferred device layer 602. The epitaxial layer may be deposited ontothe entire single crystal semiconductor device layer 602, or,alternatively, onto only a portion of the single crystal semiconductordevice layer 602. The surface of the single crystal semiconductor devicelayer 602 may comprise an oxide layer. For example, a silicon oxidelayer forms on a silicon surface when it is exposed to air at roomtemperature and generally has a thickness of from about 10 to about 15Å. Preferably, the oxide layer is removed from the surface of the singlecrystal semiconductor device layer 602 before the epitaxial layer isdeposited onto the surface. A deposited epitaxial layer may comprisesubstantially the same electrical characteristics as the underlyingsingle crystal semiconductor device layer 602. Alternatively, theepitaxial layer may comprise different electrical characteristics as theunderlying single crystal semiconductor device layer 602. An epitaxiallayer may comprise a material selected from the group consisting ofsilicon, silicon carbide, silicon germanium, gallium arsenide, galliumnitride, indium phosphide, indium gallium arsenide, germanium, andcombinations thereof. Depending upon the desired properties of the finalintegrated circuit device, the epitaxial layer may comprise a dopantselected from among boron (p type), gallium (p type), aluminum (p type),indium (p type), phosphorus (n type), antimony (n type), and arsenic (ntype). The resistivity of the epitaxial layer may range from 1 to 50Ohm-cm, typically, from 5 to 25 Ohm-cm. In some embodiments, theepitaxial layer may have a thickness between about 10 nanometers andabout 20 micrometers, between about 20 nanometers and about 3micrometers, such as between about 20 nanometers and about 2micrometers, such as between about 20 nanometers and about 1.5micrometers or between about 1.5 micrometers and about 3 micrometers.

The epitaxial deposition preferably is carried out by chemical vapordeposition. Generally speaking, chemical vapor deposition involvesexposing the surface of the wafer to an atmosphere comprising silicon inan epitaxial deposition reactor, e.g., a Centura reactor available fromApplied Materials. Preferably, the surface of the single crystalsemiconductor device layer 602 is exposed to an atmosphere comprising avolatile gas comprising silicon (e.g., SiCl₄, SiHCl₃, SiH₂Cl₂, SiH₃Cl,or SiH₄). The atmosphere also preferably contains a carrier gas(preferably H₂). For example, the source of silicon during the epitaxialdeposition may be SiH₂Cl₂ or SiH₄. If SiH₂Cl₂ is used, the reactorvacuum pressure during deposition preferably is from about 500 to about760 Torr. If, on the other hand, SiH₄ is used, the reactor pressurepreferably is about 100 Torr. Most preferably, the source of siliconduring the deposition is SiHCl₃. This tends to be much cheaper thanother sources. In addition, an epitaxial deposition using SiHCl₃ may beconducted at atmospheric pressure. This is advantageous because novacuum pump is required and the reactor chamber does not have to be asrobust to prevent collapse. Moreover, fewer safety hazards are presentedand the chance of air or other gases leaking into the reactor chamber islessened.

During the epitaxial deposition, the temperature of the wafer surfacepreferably is ramped to and maintained at a temperature sufficient toprevent the atmosphere comprising silicon from depositingpolycrystalline silicon on the surface. Generally, the temperature ofthe surface during this period preferably is at least about 900° C. Morepreferably, the temperature of the surface is maintained in the range ofbetween about 1050 and about 1150° C. Most preferably, the temperatureof the surface is maintained at the silicon oxide removal temperature.

The rate of growth of the epitaxial deposition preferably is from about0.5 to about 7.0 μm/min. A rate of about 3.5 to about 4.0 μm/min may beachieved, for example, by using an atmosphere consisting essentially ofabout 2.5 mole % SiHCl₃ and about 97.5 mole % H₂ at a temperature ofabout 1150° C. and an absolute pressure of up to about 1 atm.

In some applications, the single crystal semiconductor device layer 602comprises an epitaxial layer which imparts electrical properties. Insome embodiments, the epitaxial layer is lightly doped with phosphorous.Therefore, the ambient for epitaxial deposition comprises phosphorouspresent as a volatile compound, such as, for example, phosphine, PH₃. Insome embodiments, the epitaxial layer can contain boron. Such a layermay be prepared by, for example, including B₂H₆ in the atmosphere duringthe deposition.

Epitaxial deposition typically requires a post-epi cleaning stepfollowing epitaxial deposition to remove byproducts formed during theepitaxial deposition. This step is used to prevent time-dependent haze,which results if such byproducts react with air. In addition, manypost-epi cleaning techniques tend to form a silicon oxide layer on theepitaxial surface which tends to passivate (i.e., protect) the surface.The epitaxial wafers of the present invention may be cleaned by methodsknown in the art.

VIII. Structures

The finished SOI wafer comprises the semiconductor handle substrate 100comprising isolation region 400, the dielectric layer 500 (e.g., buriedoxide layer), and the semiconductor device layer 602, and, optionally,an epitaxial layer, may then be subjected to end of line metrologyinspections and cleaned a final time using typical SC1-SC2 process.

The present invention provides an SOI structure that eliminates thebipolar coupling between RF devices with trench isolation extended intothe handle substrates with amorphous regions. This is achieved by a thinisolation region in the handle substrate near its surface. The isolationlayer may be based on either p/n junction or dielectric orsemi-insulating material. See FIGS. 9A and 9B, which depicts a structurecomprising an isolation region comprising a p/n junction provided by aN+ buried layer. See FIGS. 10A and 10B, which depicts a structurecomprising an isolation region comprising a dielectric layer.

FIGS. 9A and 9B depict an SOI structure 702 comprising an isolationregion 714 based on an N+ layer. FIG. 9A depicts of an HR-SOI structure702 comprising a depletion region 706 and n-type inversion layer 708. Asshown in FIG. 9A, the HR-SOI structure 702 comprises a semiconductorhandle substrate 704, a buried oxide layer 710, and a semiconductordevice layer 712. The semiconductor device layer 712 may be fabricatedto comprise RF devices 720, 722. See FIG. 9B. The HR-SOI structure 702comprises an amorphous region 726 between RF devices 720, 722. Positivecharges in the buried oxide layer 710 induce deplete holds in thedepletion region 706 and induce n-type charge inversion 708. The HR-SOIstructure 702 is prepared according to the method of the presentinvention to comprise an isolation region 714 based on an N+ layer.

With reference to FIGS. 9A and 9B, during a dynamic process of RF device720, 722 operation, at a particular instance, a voltage potential of adevice 720 is altered to negative, while another device 722 is biasedpositively. The n-type inversion layer 708 is a readily available sourceof electrons. Under those devices 720 with negative bias, the n-typeinversion layer 708 injects electrons into the depletion layer. Underpositively biased devices 722, an electric field is established suchthat the related-type inversion layer 708 collects electrons from thedepletion layer 706. The thin isolation layer 714, based on N+ buriedlayer, in the handle substrate 704 near its surface eliminates thebipolar coupling 728 between RF devices with trench isolation.

FIGS. 10A and 10B depict an SOI structure 802 comprising an isolationregion 814 based on a dielectric layer. FIG. 10A depicts of an HR-SOIstructure 802 comprising a depletion region 806 and n-type inversionlayer 808. As shown in FIG. 10A, the HR-SOI structure 802 comprises asemiconductor handle substrate 804, a buried oxide layer 810, and asemiconductor device layer 812. The semiconductor device layer 812 maybe fabricated to comprise RF devices 820, 822. See FIG. 10B. The HR-SOIstructure 802 comprises an amorphous region 826 between RF devices 820,822. Positive charges in the buried oxide layer 810 induce deplete holdsin the depletion region 806 and induce n-type charge inversion 808. TheHR-SOI structure 802 is prepared according to the method of the presentinvention to comprise an isolation region 814 based on a dielectriclayer.

With reference to FIGS. 10A and 10B, during a dynamic process of RFdevice 820, 822 operation, at a particular instance, a voltage potentialof a device 820 is altered to negative, while another device 822 isbiased positively. The n-type inversion layer 808 is a readily availablesource of electrons. Under those devices 820 with negative bias, then-type inversion layer 808 injects electrons into the depletion layer.Under positively biased devices 822, an electric field is establishedsuch that the related-type inversion layer 808 collects electrons fromthe depletion layer 806. The thin isolation layer 814, based on adielectric layer, in the handle substrate 804 near its surfaceeliminates the bipolar coupling 828 between RF devices with trenchisolation.

Having described the invention in detail, it will be apparent thatmodifications and variations are possible without departing from thescope of the invention defined in the appended claims.

When introducing elements of the present invention or the preferredembodiments(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above products and methodswithout departing from the scope of the invention, it is intended thatall matter contained in the above description and shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A multilayer structure comprising: a singlecrystal semiconductor handle substrate, wherein the single crystalsemiconductor handle substrate comprises two major, generally parallelsurfaces, one of which is a front surface of the single crystalsemiconductor handle substrate and the other of which is a back surfaceof the single crystal semiconductor handle substrate, a circumferentialedge joining the front and back surfaces of the single crystalsemiconductor handle substrate, a central plane between the frontsurface and the back surface of the single crystal semiconductor handlesubstrate, and a bulk region between the front and back surfaces of thesingle crystal semiconductor handle substrate, wherein the singlecrystal semiconductor handle substrate has a bulk resistivity of atleast about 500 Ohm-cm; an isolation region in interfacial contact withthe front surface of the single crystal semiconductor handle substrate,wherein the isolation region comprising comprises an arsenic-dopedsemiconductor layer, has a thickness between about 50 angstroms andabout 10,000 angstroms, and has a resistivity between about 1×10¹²Ohm-cm and about 1×10¹⁴ ohm cm Ohm-cm; a semiconductor layer ininterfacial contact with the isolation region, wherein the semiconductorlayer comprises a polycrystalline semiconductor, an amorphoussemiconductor, or a single crystal semiconductor, wherein thepolycrystalline semiconductor, the amorphous semiconductor, or thesingle crystal semiconductor is selected from the group consisting ofsilicon (Si), silicon germanium (SiGe), silicon doped with carbon (SiC),and germanium (Ge), and the semiconductor layer has a thickness betweenabout 50 nanometers and about 4000 nanometers; a dielectric layer ininterfacial contact with the semiconductor layer; and a single crystalsemiconductor device layer in interfacial contact with the dielectriclayer.
 2. The multilayer structure of claim 1 wherein the bulkresistivity of the single crystal semiconductor handle substrate isbetween about 500 Ohm-cm and about 100,000 Ohm-cm.
 3. The multilayerstructure of claim 1 wherein the bulk resistivity of the single crystalsemiconductor handle substrate is between about 2000 Ohm-cm and about10,000 Ohm-cm.
 4. The multilayer structure of claim 1 wherein the bulkresistivity between about 3000 Ohm-cm and about 10,000 Ohm-cm.
 5. Themultilayer structure of claim 1 wherein the bulk resistivity of thesingle crystal semiconductor handle substrate is between about 3000Ohm-cm and about 5,000 Ohm-cm.
 6. The multilayer structure of claim 1wherein the thickness of the isolation region is between about 200angstroms and about 10,000 angstroms.
 7. The multilayer structure ofclaim 1 wherein the thickness of the isolation region is between about300 angstroms and about 1000 angstroms.
 8. The multilayer structure ofclaim 1 wherein the thickness of the isolation region is between about50 angstroms and about 200 angstroms.
 9. The multilayer structure ofclaim 1 wherein the isolation region comprises arsenic-doped silicon.10. The multilayer structure of claim 1 wherein the semiconductor layercomprises polycrystalline silicon.
 11. The multilayer structure of claim1 wherein the semiconductor layer comprises amorphous silicon.
 12. Themultilayer structure of claim 1 wherein the semiconductor layercomprises single crystal silicon.
 13. The multilayer structure of claim1 wherein the dielectric layer comprises a material selected from thegroup consisting of silicon dioxide, silicon nitride, siliconoxynitride, hafnium oxide, titanium oxide, zirconium oxide, lanthanumoxide, barium oxide, and a combination thereof.
 14. The multilayerstructure of claim 1 wherein the single crystal semiconductor devicelayer comprises a radiofrequency device.
 15. The multilayer structure ofclaim 1 wherein the single crystal semiconductor handle substrate isp-type and has the bulk resistivity between about 1000 Ohm-cm and about6000 Ohm-cm.
 16. The multilayer structure of claim 1 wherein thedielectric layer comprises at least two insulating layers, eachinsulating layer selected from the group consisting of silicon dioxide,silicon nitride, and silicon oxynitride.